In a steam-power generation plant, a high pressure turbine is combined with an intermediate pressure turbine and a low pressure turbine in many cases. The high pressure turbine is rotated by main steam. The intermediate pressure turbine and the low pressure turbine are rotated also by the main steam which has passed through the high pressure turbine. In the low pressure turbine in which steam pressure is low, temperature and pressure of the steam lower during an expansion process of the steam in a low-pressure stage thereof, and a part of the steam condenses into moisture. Influence of the moisture on the steam turbine will be described below with reference to the drawings.
FIG. 11 is a view showing a turbine nozzle 101 and a turbine rotating blade 102 at the final stage of the low pressure turbine, both being viewed from a meridian plane of the low pressure turbine. The nozzle 101 is supported by a diaphragm inner ring 103 and a diaphragm outer ring 104. The turbine rotating blade 102 is planted on a turbine rotor 105. A rotating blade cover 106 is arranged on the upper end of the turbine rotating blade 102. This rotating blade cover 106 connects in contact with another rotating blade covers 106 adjacent thereto to suppress vibration of the tip of the rotating blade 102. The rotating blade cover 106 also prevents steam from flowing out of a blade row of turbine rotating blades 102.
In FIG. 11, the turbine nozzle 101 shows a leading edge thereof and the turbine rotating blade 102 shows a suction side thereof when viewed on the paper. Steam condenses on the surface of the leading edge of the turbine nozzle 101 to generate moisture. The moisture attaches to the leading edge of the turbine nozzle 101 to collect, thereby forming a liquid film 107.
FIG. 12 is a view showing a section cut along the line XII-XII in FIG. 11. The liquid film 107 reaches a rear edge 108 of the turbine nozzle 101 and changes into water droplets 109 to fly off from the back edge 108. The arrow denotes a scattering direction of the water droplets 109 in FIG. 12. On the scattering, steam energy is used for acceleration of the water droplets 109 and is, therefore, consumed.
The water droplets 109 cannot move completely into a steam flow as a result of inertia thereof. This event causes the water droplets 109 to collide with the suction side 110 of the turbine rotating blade 102 which is rotating. The collision of the water droplets 109 with the suction side of the turbine rotating blade 102 serves as a retarding force against the rotation of the turbine rotating blade 102, and reduces turbine efficiency. The turbine rotating blade 102 is likely to be eroded because the water droplets 109 attach to the suction side 110 of the turbine rotating blade 102.
As described above, the moisture attaching to the turbine rotating blade 102 has an adverse effect on efficiency and reliability of a turbine. On the other hand, there is known a steam turbine provided with a structure to remove attached moisture. Such a device will be described below with reference to FIGS. 13 and 14.
FIG. 13 is a sectional view showing a turbine nozzle 101 being viewed from a meridian plane thereof. A device shown in FIG. 13 is provided to the turbine nozzle 101 of a hollow structure having a slit 111 on a front-side surface thereof so that moisture attached to the leading edge surface is introduced into the inside of the turbine nozzle 101 via the slit 111.
FIG. 14 is a sectional view showing the turbine rotating blade 102 being viewed from a meridian plane thereof. A device shown in FIG. 14 arranges grooves 112 extending in the longitudinal direction of the rotating blade on a suction side surface 110 of the rotating blade 102 so that moisture attached to the grooves 112 is collected to a drain pocket 113 formed inside the diaphragm outer ring 104 by centrifugal force of the turbine rotating blade 102. FIG. 15 is a perspective view of the turbine rotating blade 102 shown in FIG. 14. As shown in FIG. 15, the rotating blade cover 106 is arranged so that the end face thereof coincides approximately with a front outside-edge of the suction side surface 110 of the turbine rotating blade 102 and the grooves 112 are arranged from the suction side surface 110 of the turbine rotating blade 102 to the end face thereof. As another embodiment, there is disclosed a configuration which provides the rotating blade cover 106 with a moisture ejection hole connecting to the grooves 112.
The device shown in FIG. 13 is provided with the slit 111 on the turbine nozzle 101 to remove moisture. However, such a device is likely to take in not only moisture but also steam via the slit 111 into the inside of the turbine nozzle 101. The steam flowed into the inside of the turbine nozzle 101 may have no contribution to rotation of a turbine, thereby reducing turbine efficiency. The turbine nozzle 101 is needed to be hollow and is therefore more difficult to manufacture than a normal turbine nozzle 101.
On the other hand, the device shown in FIGS. 14 and 15 provides the turbine rotating blade 102 with the grooves 112 to collect moisture into the drain pocket 113. The device requires nothing other than forming the grooves 112 on the turbine rotating blade 102. Therefore, the turbine rotating blade 102 having such a device is easy to manufacture. A small amount of steam flows outside the rotating blade cover 106. Therefore, steam flowing into the drain pocket 113 is less than steam flowing into the slit 111. In other words, the device providing the turbine rotating blade 102 with the grooves 112 has less impact on turbine efficiency than the device providing the turbine rotating blade 102 with a hollow and the slit 111.
As mentioned above, the device providing the turbine rotating blade 102 with the grooves 112 has less impact on turbine efficiency than the device providing the turbine rotating blade 102 with a hollow and the slit 111. However, steam is likely to flow out of the grooves 112 to the outside of the rotating blade cover 106.
The nearer the final stage of the turbine rotating blade 102, the more moisture attaching to the turbine rotating blade 102 is. When the number of the grooves 112 is increased to deal with an increase in moisture, the number of the grooves 112 passing through the connected rotating blade covers 106 or the number of exhaust nozzles for water droplets is also increased. This increases an amount of steam flowing out of the rotating blade cover 106.
It is also necessary to enlarge the entrance width of the drain pocket 113 in connection with increasing the number of the grooves 112. When enlarging the entrance width of the drain pocket 113, the amount of the steam flowing into the drain pocket 113 also increases. When the drain pocket 113 is located on the vertically upper side of the turbine rotating blade 102, moisture is likely to collide with the inside wall of the drain pocket 113 having a wide entrance and to reflect on the inside wall. In such a case, the moisture is likely to fall from the wide entrance to the side of the turbine rotating blade 102.